Ceramic and ceramic composite material components and ceramic coated components are attractive candidate materials for improved efficiency power systems over their metal counterparts because of the lighter weights and higher strengths at elevated temperatures of the ceramic-based components. For example, monolithic silicon nitride vanes and turbine blades and silicon carbide composite combustor liners are currently being evaluated in high performance gas turbine engines. Highly stressed areas of ceramic components, such as turbine blades, include the near-surface regions, particularly the blade trailing edges. Accumulation of elevated temperature damage and microstructural changes over time in these near surface regions may lead to premature failure.
When ceramic and ceramic composite materials and ceramic coatings are loaded at temperatures above approximately one-third of their melting/ablation temperatures, these materials and coatings undergo creep phenomena. These creep phenomena may include: development of cavities or voids along grain boundaries, development of voids in grain boundary glassy phases, formation of wedge cracks at grain boundary triple points, linkage of cavities to form grain facet sized cracks which then link to form microcracks, and linkage of microcracks to form macrocracks until failure occurs when a critical flaw size is reached. Microstructural changes such as grain growth, phase changes, dissolution and precipitation phenomena, oxidation, scaling and spalling may also occur. Coatings subjected to thermal and mechanical stresses may develop internal cracks as well as delaminations at the coating substrate interfaces, leading ultimately to spalling.
Structural ceramics, e.g., silicon nitride and silicon carbide ceramic and ceramic composite materials, are increasingly being used for high temperature gas turbine applications in not only vanes and turbine blades, but also combustor liners because of their high thermal stabilities and elevated temperature strengths. In these types of applications, the critical regions of a component experiencing the highest stresses are frequently the surface and near-surface regions to a depth on the order of 200-300 microns. Increased depths to approximately 500 microns are also of interest in the structural analysis of these ceramic-based materials. The presence of large stresses on the surface and in near-surface regions can lead to the development of creep cavities and wedge cracks which ultimately link to form microcracks until a dominant microcrack results in rupture. The detection of damage accumulation in structural ceramics, particularly Si.sub.3 N.sub.4 and SiC creep tested in air is extremely difficult. Periodic surface inspection of Si.sub.3 N.sub.4 components undergoing high temperature loading reveals that oxidation products and the formation of glassy phases frequently obscure the presence of surface flaws. Indeed, attempts to use hardness indentations to measure the development of creep strains in creep flexure bars have proved unsuccessful due to glassy phases obscuring the reference indents during the creep test.
The present invention addresses the aforementioned limitations of the prior art by allowing for the periodic "imaging" of a ceramic component in its operating environment, i.e., as installed in an operating component of a machine, and allows the material at the surface as well as below the surface to be assessed for damage. This inventive method and apparatus is particularly adapted for use in detecting defects and microstructural changes in ceramic and ceramic composite materials in bulk form as well as in ceramic coatings such as used in nozzles, vanes, rotor blades and combustor lines used in high temperature gas turbine applications.